1. Field of the Invention
The present invention relates to a fuel cell battery with afterburning at the periphery of a cell stack.
2. Description of the Prior Art
An apparatus with high temperature fuel cells is known from EP-A 0 580 918 which comprises a centrally symmetric cell stack and a heat insulating envelope. An afterburner chamber lies between the cell stack and the envelope. Air infeed lines for the fuel cells which traverse the afterburner chamber at a large number of locations are arranged at the periphery of the stack.
Each fuel cell comprises two parts, namely a disc-shaped interconnector and a so-called PEN (derived from: Positive electrode/solid Electrolyte/Negative electrode). The PEN is an electrochemically active element which is arranged in the form of a thin solid electrolyte plate with two electrode layers between interconnectors. The interconnector produces electrical connections between the PEN electrodes of adjacent cells. It contains a passage system for gaseous fluids, via which on the one hand a fuel gas is transported from a central entry point along the PEN to the periphery of the cell stack and on the other hand air or a gas containing oxygen is transported from the air spaces to the centerxe2x80x94for the purpose of a further heating up of the airxe2x80x94and from there back along the PEN to the periphery.
The air infeed lines to the cells cause high costs as a result of their large number. The object of the invention is to create a fuel cell battery which is more economical with respect to the air infeed into the fuel cells.
The fuel cell battery is operated with an afterburning at the periphery of a cell stack. Each cell of the battery has at least one entry point for air or another gas containing oxygen. The afterburning is provided inside a ring-shaped space about the cell stack. The named entry points are communicatingly connected as a totality or in each case group-wise via at least one air space. The air space extends axially along the cell stack and stands in direct contact with the cell stack. Each air space is separated from an afterburner chamber, which likewise forms a space which communicates axially along the cell stack, by at least one wall.
In one embodiment the interconnectors are formed in two layers. Each layer of the interconnector consists of a shaped and coated basic body which has a temperature expansion behavior which is at least approximately similar to that of the PEN and which is largely sintered from a metallic powder into the form of a finished part. Two layers of this kind are for example assembled to a unit through soldering. The passages of the interconnectors, on both sides of which in each case a PEN covers over the passages which are open there, have for example a spiral shape, and indeed in such a manner that during the operation of the battery the PEN electrodes are largely uniformly brushed over by the gaseous fluids (fuel gas and air or gas containing oxygen).
The interconnectors and PENs are stacked up as single partsxe2x80x94forming an alternating sequencexe2x80x94to a stack. This cell stack is pressed together by means of tensioning means, for example draw bars, in the direction of the stack axis. Discretely arranged openings for the entry or departure respectively of the gaseous fluid are located at the periphery of the cell stack. The positions of these openings are determined by the design of the passages and the conducting in and out of the fluid which is provided in accordance with the invention. The openings can be correspondingly pre-shaped in the sintered interconnector layers. The manufacture of the interconnectors 22 by means of sintering enables a very high dimensional precision. Contacts which are sufficiently impervious with respect to a radial passage of air or gas respectively are therefore possible at the joint locations between the interconnectors 22 and the PEN plates in the cell stack 2.
As already in the known battery of EP-A 0 580 918, the envelope can be designed as a heat insulating system so that the envelope takes over the role of an external recuperator in addition to its heat insulating function: instead of the air required for the electrochemical processes in the cells being pre-heated in a separate external recuperator, the air is used as a heat sink in that the heat which flows away out of the cell stack is at least partly captured in the envelope and conducted back to the reaction location.
The known envelope is designed in a plurality of layers; it has a passage system for the air flow. Between the outer wall, which forms a first layer of the envelope, and the inner parts of the envelope there lies a first cavity in which a cooling of the envelope by the air takes place. In the passage system, which adjoins at the first cavity, there results a further heating of the air. Instead of or in addition to the passages, porous, air-permeable parts can also be built in in the envelope.